1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device means any device which can function utilizing semiconductor characteristics, and includes in its category an electrooptic device, a semiconductor circuit, and an electronic device.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (the transistor is also referred to as a thin film transistor (TFT)). The transistor has been applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). Although a silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor, an oxide semiconductor has been attracting attention as an alternative material.
For example, a transistor whose active layer uses an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) with an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Document 1).